Why is honey the only food that does not go bad?

Pure honey does not go bad. Most honey is a supersaturated with sugar. Sugar being hygroscopic, its attractiveness to moisture makes honey a dehydrating agent. Most microorganisms do not grow in honey because of its low water activity. Molds and bacteria that land on the honey lose their own moisture(desiccate) to the honey, Thus their growth cycle is compromised and spoilage is reduced.
Over the time, honey may get crystallized. Crystallized honey can be re-liquefied by heating. You should never leave a jar of honey standing open. The supersaturated sugar solution will absorb moisture from the air and gradually become weaker, losing its anti-bacterial properties.

Why does mouth water when we see or smell delicious food?

When we see or smell good food, the brain registers that food is coming, and it instructs the salivary gland underneath our tongue to produce saliva in anticipation. Saliva helps break down the food as we chew it and makes it softer as it enters the gullet. As this is a subconscious reaction, it will occur even if we just see or smell the food.

Why do we get lumps in our throats when we hold back tears or are about to cry?

The ‘lump’ is actually not a lump at all but a counter-reaction to the body’s automatic nervous system. When humans are exposed to stressful situations,  the body automatically increases blood flow to vital organs and muscles. This is our typical ‘fight or flight’ biological reaction to things. Consequently, in order to get more oxygen to the body, one biological response is to expand the glottis in your throat.
But, being the social creatures we are, there are times when we try to suppress the urge to cry and often constrict or stop breathing altogether. Hence we have a battle between the body’s natural response to stress by widening the muscle in the throat to get more oxygen, and one’s wish to avoid shedding tears where we often stop breathing and thus constrict the muscle in our throat.  Hence, the vocal chords are fighting themselves to both compress and expand at the same time - and that's when we experience the tension as a lump. 

What does 20/20 vision mean?

During an eye test, eye doctors use eye charts to measure how well you see in the distance, compared with other human beings. The eye chart displays letters of progressively smaller size and it compares a person's vision to the that of the average for the population. "Normal" vision is 20/20. This means that the test subject sees the same line of letters at 20 feet that a normal person sees at 20 feet. 20/40 vision means that the test subject see as 20 feet what a normal person sees at 40 feet.
20/20 means that the person whose vision is being tested is normal.

What is blue moon?

A Blue Moon is commonly the name given to the second full moon in a calendar month. Since a full moon occurs every 29 1/2 days, if there is a full moon on the 1st or 2nd day of a month, there is a good chance there will be a second full or blue moon that month. Blue moons happen every two to three years. The moon looks like a regular full moon and not blue in color.
'Once is a blue moon' is frequently used expression which means ‘not very often,’ or ‘very rarely'.

Why do rain clouds float inspite of carrying water?

Rain clouds have bigger volume but very small mass,hence very low density (lower than that of air) because of which they float. Clouds generally form, survive and grow in air that is moving upward. Rising air expands as the pressure on it decreases, and that expansion into thinner, high-altitude air causes cooling. Enough cooling eventually makes water vapor condense, which under gravity falls as rain. So, even though typical clouds do contain a lot of water, this water is spread out for miles in the form of tiny water droplets or crystals, which are so small that the effect of gravity on them is negligible. 

Does a blood transfusion change our DNA?

Red blood cells, the primary component in transfusions, have no nucleus and no DNA. When blood is donated, it is processed in a number of ways but one important way is 'leukodepletion' - which is the removal of the white blood cells. If this wasn't done, the donated white blood cells would attack your body's tissues and cause inflammation.
A transfusion is supposed to give someone enough blood to last until they can make their own. So while it is theoretically possible to have other's  white blood cells DNA in a sample, those cells will eventually be eliminated from the body. 

What is the dot over letters 'i' and 'j' called?

Usually in many languages, specific characters are added to an alphabet to denote a particular accent.  This a mark added to a letter that is meant to signal a change in either the sound or meaning of a character.
The small distinguishing mark you see over a lowercase 'i' and a lowercase 'j' is called a tittle- which seems like a combination of tiny and little.  It was added to the letter i in the Middle Ages to distinguish the letter from adjacent vertical strokes. Tittle is also referred as a superscript dot or a jot.  

Why are umbrellas mostly black in color?

Basically, the umbrella was initially meant for rainy seasons. In the olden days umbrellas were made of thick cotton cloth. When they got wet in the rains, they would take a long time to dry up. It was found that umbrellas made of black cloth dried faster than those made of cloth of other colors. So it became a standard practice to use black cloth in the making of umbrellas. Black cloth dries faster because black or dark surfaces absorb more heat from the surroundings than lighter surfaces and as a result the evaporation process is faster.
Today, umbrellas are made of fabrics that do not soak water, and so, as drying is no longer a problem, umbrellas come in all shades and colors.

Why does blowing on a candle extinguish it?

To put out a fire you either need to remove the fuel, oxygen or heat. When you blow on a candle the movement of the air across the wick removes the heat from the flame. Cooler air takes its place. This lowers the temperature of the burning wax to below its ignition point. The wax stops burning and the candle goes out.